Patent GB-1148261-A (GB)
Title: Apparatus for the manufacture of resistors
Inventor: WELLARD CHARLES LEROY
IPC: H01C17/245
Filed: 1966-03-18
Abstract: 1,148,261. Grinding; blasting. AMERICAN COMPONENTS Inc. 18 March, 1966, No. 11893/66. Heading B3D. [Also in Division H1] A helical groove is cut by a grinding wheel or by sand-blasting in the metal film or other electrically conductive coating of a resistor blank and the pitch of the groove is adjusted either in accordance with a signal derived from a resistance bridge which measures the resistance of the blank at the start of the machining operation or in accordance with one measuring variation in the resistance of the blank during cutting. Machining is stopped automatically when the resistance of the blank reaches the desired final value. The pitch of the helical groove cut by a grinding wheel 77 in the metal film or other electrically conductive coating of resistance element blank 19 is adjusted, so that finished resistors of a specified value may be produced from unsorted varying blanks, in accordance with a signal derived from bridge 11 at the start of the machining operation, and machining is stopped when bridge 53 shows that the resistor has reached the desired value. Decade resistor boxes 15, 85 are set to the value required and bridge 11 (which is preferably A.C.) has blank 19 connected to it by switch 29. The signal which passes between points 35 and 37 is amplified and used to control a servo which adjusts variable resistor 17 (the direction of movement depending on the polarity of the signal) until bridge 11 is balanced and also to adjust the speed control 49 of leadscrew motor 51, which moves table 83 and blank 19 parallel to their length during cutting. The speed of rotation of blank 19 is constant, so the pitch of the helical groove in the finished resistance element depends on the resistance of the blank before machining and on the final value required. When the speed controller 49 of lead screw motor 51 is set switch 29 is manually or automatically operated to connect blank 19 as a leg of bridge 53 during machining. As long as bridge 53 is unbalanced and a signal is passed between points 57, 59 thyratron 63 remains in a "cut-off" state, but when the resistance of blank 19 has risen to its required final value bridge 53 balances, the signal ceases and the thyratron becomes conductive. Solenoids 64, 68 then operate, respectively tilting table 83 and the now-complete resistance element away from grinding wheel 77 and disconnecting table 83 from lead screw 52. Bridges 11, 43 may be combined, with a single decade resistor box 311 and a variable resistor which can be converted into a fixed one by opening a shorting lead 317, 319a. Continuous monitoring of the resistance element blank as it is cut (so that the speed of the leadscrew motor may be adjusted during cutting to compensate for, e.g. variations in the resistivity of the metal film) can be obtained by substituting a pair of bridges, Fig. 2, for bridge 53. Resistor blank 211 is placed in bridge 253 and its desired value in the decade resistor box. The signal resulting from the lack of balance of the bridge is passed to a high impedance device connected to a resistor equal to the value of resistor 223 when its adjustable arm 224 has been moved the distance of the table travel. Bridge 225, containing these, is isolated from bridge 253 by the high impedance device. As the helical pattern is cut the signal across bridge 253 is reduced and as the movable table with the resistance element blank moves resistance 223 is altered. The linear characteristic of resistor 223 matches the travel of the table. If the resistance element 211 departs from a linear variation in resistance, as it is cut, a signal is produced across bridge 225 and the speed of the lead screw motor adjusted to balance bridge 225 again.